diff --git a/sys/dev/rtwn/if_rtwn.c b/sys/dev/rtwn/if_rtwn.c
--- a/sys/dev/rtwn/if_rtwn.c
+++ b/sys/dev/rtwn/if_rtwn.c
@@ -331,6 +331,11 @@
 	    "ht40", CTLFLAG_RDTUN, &sc->sc_ht40,
 	    sc->sc_ht40, "Enable 40 MHz mode support");
 
+	sc->sc_ena_tsf64 = 0;
+	SYSCTL_ADD_INT(ctx, SYSCTL_CHILDREN(tree), OID_AUTO,
+	    "ena_tsf64", CTLFLAG_RWTUN, &sc->sc_ena_tsf64,
+	    sc->sc_ena_tsf64, "Enable/disable per-packet TSF64 reporting");
+
 #ifdef RTWN_DEBUG
 	SYSCTL_ADD_U32(ctx, SYSCTL_CHILDREN(tree), OID_AUTO,
 	    "debug", CTLFLAG_RWTUN, &sc->sc_debug, sc->sc_debug,
diff --git a/sys/dev/rtwn/if_rtwn_rx.c b/sys/dev/rtwn/if_rtwn_rx.c
--- a/sys/dev/rtwn/if_rtwn_rx.c
+++ b/sys/dev/rtwn/if_rtwn_rx.c
@@ -285,8 +285,18 @@
 		rxs.c_pktflags |= IEEE80211_RX_F_FAIL_FCSCRC;
 
 	rxs.r_flags |= IEEE80211_R_TSF_START;	/* XXX undocumented */
-	rxs.r_flags |= IEEE80211_R_TSF64;
-	rxs.c_rx_tsf = rtwn_extend_rx_tsf(sc, stat);
+
+	/*
+	 * Doing the TSF64 extension on USB is expensive, especially
+	 * if it's being done on every MPDU in an AMPDU burst.
+	 */
+	if (sc->sc_ena_tsf64) {
+		rxs.r_flags |= IEEE80211_R_TSF64;
+		rxs.c_rx_tsf = rtwn_extend_rx_tsf(sc, stat);
+	} else {
+		rxs.r_flags |= IEEE80211_R_TSF32;
+		rxs.c_rx_tsf = le32toh(stat->tsf_low);
+	}
 
 	/* Get RSSI from PHY status descriptor. */
 	is_cck = (rxs.c_pktflags & IEEE80211_RX_F_CCK) != 0;
diff --git a/sys/dev/rtwn/if_rtwnvar.h b/sys/dev/rtwn/if_rtwnvar.h
--- a/sys/dev/rtwn/if_rtwnvar.h
+++ b/sys/dev/rtwn/if_rtwnvar.h
@@ -172,6 +172,7 @@
 	device_t		sc_dev;
 
 	int			sc_ht40;
+	int			sc_ena_tsf64;
 	uint32_t		sc_debug;
 	int			sc_hwcrypto;
 	int			sc_ratectl_sysctl;